The present invention relates to apparatus for pre-piercing preheated billets which are to be extruded at a later time, and a method for pre-piercing the preheated billets.
Known pre-piercing devices operate as follows. First, a billet, made of metal, is heated to a temperature suitable to permit piercing of the billet. The billet, now preheated, is then placed in a container and a mandrel bar is inserted through the billet, causing the billet to deform within the container so as to accommodate the mandrel bar. The pre-pierced billet so formed is then removed and extruded in a subsequent step.
Existing pre-piercing technology provides piercing along only a portion of the length of the billet. In the known technology, a slug is left inside the billet and the hole formed by the mandrel bar does not extend throughout the entire length of the billet. The slug is then removed in a subsequent operation.
A problem exists with this existing pre-piercing technology, in that the shearing out of the slug from the inside of the billet can cause structural damage to the billet, or can produce internal stresses, that can adversely affect the quality of the billet when subsequently subjected to a final extrusion operation.
There are several disadvantages associated with the known pre-piercer devices. First, during operation, these pre-piercer devices must include delays between consecutive strokes of the mandrel bar to permit loading of another billet to be pre-pierced and to permit unloading of a billet which has already been pre-pierced along a portion of its length by the mandrel bar. Therefore, the throughput of billets in the known pre-piercing devices is limited by the time required for the steps of loading and unloading the billets.
Another problem in the art is the provision of good concentricity of the opening formed in the pre-pierced billet. Gaps existing between the billet and the container prior to pre-piercing are detrimental to the concentricity of the opening formed in the pre-pierced billet by the mandrel bar.
It is another problem in the known pre-piercer devices to permit changeover to another billet size without reconfiguring the pre-piercer device and without requiring a lengthy shutdown for replacement of parts.
It is still another problem in the art to reduce deflection of the mandrel bar as the mandrel bar pushes its piercing tip into the billet. Deflection of the mandrel bar cause eccentricity not only in the shape but also in the mechanical properties of the resulting pre-pierced billet. Deflection of the mandrel bar during pre-piercing can be caused by variations in the mechanical properties of the billet to which is to be pre-pierced. For example, uneven heating of the billet prior to the pre-piercing operation can generate hard or soft spots in the billet. Since the piercing tip of the mandrel bar will tend to follow the path of least resistance, restrained only by its own mechanical rigidity and by any support or guide that is provided, the mandrel bar will therefore tend to deflect during the pre-piercing operation.
Additionally, it is a problem in the prior art to remove a slug from a billet during the pre-piercing process.
It is still another problem in the art to selectively control the slug thickness. In view of various practical considerations involving the desired qualities of the resulting pre-pierced billet, the slug has a preferred thickness which varies depending upon the billet diameter, billet length, and composition of the billet.